This disclosure relates to apatite materials, and in particular to coatings comprising apatite.
Synthetic apatites, including hydroxyapatite (HA), Ca10(PO4)6(OH)2, fluorapatite Ca10(PO4)6F2, or hydroxyfluorapatite Ca10(PO4)6(OH)2-xFx (x<1) have a similar composition to biological apatites (Ca10-x(HPO4)x(OH)2-x), principle components of hard tissues such as bone. Due at least in part to their excellent osteoconductivity, apatites have been used as a coating on metallic implants or prostheses (i.e., prosthetic hips) for biological fixation. Such an apatite coating has two interfaces, one with the tissue of the subject receiving the implant and one with the implant itself. The benefits of apatite coatings, particularly in early stage fixation, include decreased fibrous tissue growth near implant surface, a decrease in pain at the site of implantation, and increased bone or other favorable tissue growth onto the surface of the apatite coated metal implant.
While apatite coatings can have positive interactions with bone and tissue, the apatite-implant interface can be a source of failure. In Biomaterials 17: 397-404, 1996, it has been reported that the failure of apatite-coated implants is due at least in part to the intrinsic properties of apatite and metallic implants, including dissolution of amorphous apatite in body fluids, thermal expansion mismatch between apatite and the metallic substrate, and corrosion at the metal-coating interface during the service lifetime of the implant. While crystalline apatite, particularly crystalline hydroxyapatite, is relatively insoluble in body fluids, amorphous apatite and soluble impurities such as calcium phosphates (i.e., Ca3(PO4)2, Ca4P2O9) and CaO can dissolve into body fluids, leading to delaminated apatite layers and cracks in the coating. Cracks can also result from the large thermal expansion coefficient difference between the apatite and the metallic substrate. For instance, hydroxyapatite has a thermal expansion coefficient of αHA=13.3×10−6/° C. and metallic alloys such as Ti6Al4V have a thermal expansion coefficient of αTI alloy=10.3×10−6/° C.
Once formed, individual cracks in the apatite-implant interface can lead to the formation of connective cracks. Cracks in the apatite-implant interface, particularly connective cracks, can allow the migration of body fluids. This fluid migration can corrode a titanium alloy substrate, thereby generating a high concentration of protons (H+) at the apatite-implant interface (Eq. 1). The concentrated H+ can dissolve the apatite coating upon contact at the apatite/Ti interface (Eq. 2), resulting in failure of implanted prostheses.Ti+2H2O→TiO2+4H++4 e−  (1)Ca10(PO4)6(OH)2+2H+→10 Ca2++6 (PO4)−3+H2O  (2)
Because there is little circulation of body fluid at the interface local area, reactions (1) and (2) can occur continuously, until the apatite/Ti interface is nearly or completely separated and large interconnected cracks are formed. When these large cracks are wetted by the surrounding body fluid, circulation of the body fluid will quickly neutralize the local protons and increase the pH to its normal biological level. Once the pH is no longer acidic, the apatite dissolution will cease; however, by this time the apatite-implant interface will have been significantly degraded.
There thus remains a need for improved apatite containing coatings for metal substrates such as prostheses or implants.